The invention relates to image masks for photolithography. More particularly, the invention relates to an image mask assembly. Even more particularly, the invention relates to image masks and image mask assemblies for writing color filters and thin film transistors for liquid crystal display panels.
Image masks are used to write patterns on photoresist films in photolithography applications. Light of a selected wave length is projected through the patterned image mask onto portions of the photoresist layer, causing those portions to react. Unreacted or reacted portions of the photoresist layer are then stripped away to leave either a positive or negative pattern. Features may be formed on the portions of the substrate underlying the stripped regions of the photoresist layer.
Image mask technology is particularly useful in forming color filters and thin film transistors on glass panels that are used in liquid crystal display (LCD) panels. LCD image masks currently include a monolithic piece of high purity fused silica onto which a patterned layer comprising chromium is deposited. Fused silica is expensive to form, and requires extensive polishing to achieve the finish needed for an image mask. Each patterned chromium layer is unique; whenever the pattern design is changed or an image mask is damaged or becomes worn, the entire image mask must be scrapped or rebuilt, and replaced.
As the size of LCD panels increases, the corresponding size of the image mask needed to write patterns on various pieces of the panel increases. Consequently, the cost of providing a fused silica image mask having a precision polished surface that is very flat and has a very low total thickness variation becomes even greater.
Other technologies, such as ink jet technologies, have been used to reduce the cost of fused silica image masks by replacing precision photolithography with direct write technologies. However, these technologies are less precise than photolithography.
Attempts have been made to substitute lower quality glasses, such as soda lime glass, for fused silica in the image mask. However, problems, such as high thermal expansion coefficients, tendency of the glass to sag, low transmission, and high concentrations of inclusions, tend to make such substitutions ineffective.